Lightly grease an 18 x 23 cm/7 x 9 in Swiss roll tin.
Cream 175 g/6 oz of the butter or margarine with the sugar until very soft, light and fluffy.
Sift together the flour and rice flour and gradually work these dry ingredients into the creamed mixture.
Press the mixture into the prepared tin and smooth the surface with the back of a warm metal spoon.
Chill for 1-2 hours.
Bake the shortbread in a moderate oven (180C, 350F, gas 4) for about 40 minutes, or until it is a pale straw colour.
Leave in the tin to cool completely.
While the shortbread is cooling, make the topping.
Melt the remaining fat, the condensed milk and syrup in a saucepan over a low heat.
Bring to the boil and boil steadily for 5 minutes, stirring continuously.
Pour the toffee over the cold biscuit base.
While the toffee is still hot, sprinkle it evenly with chocolate and allow the chocolate to melt.
Cut the shortbread into squares when the topping is set, using a hot metal knife.